The present invention relates to a simplified demand limit control and, more particularly, to a simplified demand limit control for maintaining instantaneous energy consumption of the loads within a building below a predetermined value.
Public electric utility companies are committed to provide suitably sized supply equipment to meet the needs of their customers. This equipment must be sized to not only handle the ordinary everday usage of electricity but must also be designed to meet the peak rates of electricity consumed by the utility's customers. Thus, if one customer has an abnormally high demand for a short period of time, the utility must provide heavier supply equipment to satisfy the short term high demands of that customer than would otherwise be needed to supply the demands of the other customers even though the other customers may essentially have the same average power consumption.
The utility typically recovers the cost of providing this heavier supply equipment by monitoring their customers' rate of power consumption and charging those customers which have high peak demands a surcharge which is dependent upon the maximum rate used in a given billing period. This surcharge is an added expense to the total power usage and is perceived as a part of the cost of energy because the utility has had to provide the heavier supply equipment to satisfy these higher demands. Therefore, it is not surprising that the commercial building industry's interest in energy conservation has included various routines for leveling and limiting electrical demand.
Many types of systems have been introduced in order to limit electrical consumption within a building to avoid excessive power charges. Typical systems put the decision making authority for deciding which loads to be shed or added in a central controller. In this arrangement, the central controller through a power meter monitors power consumption within a building and determines what the load within the building must be in order to maintain power consumption below a demand limit established by the user for his facility. The demand limit is selected by the user at a level low enough to minimize the surcharge imposed upon him by the utility. Such systems can either be home run wherein the central controller has a separate control line for each of the loads connected to it or can be daisy chained in such a fashion that the central controller must address each load and communicate to the load the controller's decision as to whether or not the load should be shed or added.
In either case, these systems are complex and are typically used where a great many loads are to be so controlled. Furthermore, the decision making process is done within the controller and is not done at the individual loads. However, copending application Ser. No. 512,519 filed on July 11, 1983 discloses a system in which the decision making authority is removed from a single processor and instead resides in a plurality of load controlling modules. In this system, the power consumption rate is broadcast over a communication channel and is received by each of load controlling modules. To each load controlling module is connected a plurality of loads. Each load controlling module determines based upon current power consumption whether or not it should shed a load. The modules are arranged in priority order and each load within a module is arranged in priority order. Thus, module 1 will shed its load 1 if it determines that power consumption is too high. Thereafter, module 2 will then determine whether or not it should shed its first load if power consumption still remains too high, and so on. This system is again directed to buildings which have many loads to be controlled. Moreover, such a system does not allow the individual loads to make their own decisions as to whether or not they should be shed or added.